In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of combustion gas turns the turbine by impingement against an airfoil section of the turbine blades and vanes, which turns the shaft and provides power to the compressor and fan. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forwardly.
The hotter the combustion and exhaust gases, the more efficient is the operation of the jet engine. There is thus an incentive to raise the combustion and exhaust gas temperatures. The maximum temperature of the combustion gases is normally limited by the materials used to fabricate the turbine vanes and turbine blades of the turbine, upon which the hot combustion gases impinge. In current engines, the turbine vanes and blades are made of nickel-based superalloys, and can operate at temperatures of up to about 1900-2100.degree. F.
Many approaches have been used to increase the operating temperature limit of the turbine blades and vanes to their current levels. For example, the composition and processing of the base materials themselves have been improved.
Physical cooling techniques may also be used. In one technique, internal cooling passages through the interior of the turbine airfoil are present. Air is forced through the cooling passages and out openings at the external surface of the airfoil, removing heat from the interior of the airfoil and, in some cases, providing a boundary layer of cooler air at the surface of the airfoil. In another approach, a protective layer or a metal/ceramic thermal barrier coating (TBC) system is applied to the airfoil, which acts as a substrate.
The gas turbine blade or vane is operated in a highly aggressive environment that can cause damage to the component in service. The environmental damage may be in various forms, such as particle erosion, different types of corrosion, and oxidation, and complex combinations of these damage modes, in the hot combustion gas environment. The rate of environmental damage may be lessened somewhat with the use of the protective layers. However, the various types of environmental damage are still observed, often necessitating premature replacement or repair of components after service exposure.
There is a need for an improved approach to the protection of gas turbine components such as turbine blades and vanes. The present invention fulfills this need, and further provides related advantages.